What is not suitable as a liner underneath a composite filling?

Zinc phosphate cement is not suitable as a liner underneath a composite filling because of its properties. While zinc phosphate is an excellent material for use as a base, it can be relatively technique-sensitive and is not as biocompatible as other materials typically used as liners. Liners need to provide a protective barrier between the pulp and the restorative material, contribute to the health of the dental pulp, and ideally promote reparative dentin formation.

In contrast, materials like glass ionomer cement, calcium hydroxide, and resin-modified glass ionomer are commonly used as liners because they provide dental pulp protection, have better biocompatibility, and promote healing or environmental stability in the tooth structure. They contribute to the overall health of the tooth and enhance the longevity and success of the composite restoration.